Your noise is not coming from a plugged up a/c drain tube, it is coming from air in the coolant system, take it from someone who has delt with these issues for along time.
You need to bleed the system get it full of coolant and then have it pressure tested.
Mike

 

Mike....I filled the reservoir and the noise has gone. The fluid has not dropped since I filled it last week. How do you bleed it?

 

I am interested in how to bleed the system as well. I have had the same problem.

I purchasedmy 2001 D2 a few weeks ago. After I drove off, I heard the sound, and noticed that the coolant was low. I filled it, and the sound went away. It came back, and the coolant was again low. I filled it again and the sound has not come back and the coolant is not low anymore. It has been about 2 weeks.

 

When the engine is cold, remove the bleed screw on the black plastic tee where your radiator hose join up on pot of the engine. Then fill the coolant bottle to the top and slowly keep powing until coolant starts coming out of the bleed screw, reinstall the bleed screw and it's seal, put the cap on the coolant bottle and drive it for a day or two, then top off the coolant bottle one more time.
Once this is done then it is time go have the trucks coolant system pressure tested to find the leak.
If you have any questions, send me your tele number and I'll call you.
Mike
mikeljacobs@comcast.net